Human Immunodeficiency Virus (HIV) infection of T cells by infected T cells is greatly enhanced by the formation of adhesive structures called virological synapses. Virological synapse formation is dependent on Env/CD4 interactions and results in the rapid accumulation of virus in internal endocytic compartments of the target cell. In addition to being a highly efficient mode of HIV transfer, transfer across the virological synapse is resistant to some inhibitors of cell-free infection. The specific aims of this grant are designed to test our hypothesis that virus is transferred into intracellular compartments of target cells where it subsequently undergoes maturation and fusion. This is especially significant because fusion is an early and essential step in establishing a productive infection. Enzymatic and fluorescent reporters of viral fusion in combination with inhibitor studies will be used to test this hypothesis. In addition, live cell imaging using fluorescent reports of viral fusion will provide us with important spatial information regarding where the fusion is taking place in the target cell. A better understanding of the processes that regulate cell-cell transmission of HIV may lead to the development of antiretrovirals that are more effective in vivo. PUBLIC HEALTH RELEVANCE: Transfer of HIV-1 across T cell virological synapses is a recently appreciated method of viral transfer that is both highly efficient and resistant to some inhibitors of cell-free infection. This proposal seeks to determine the mechanisms by which cell-cell transfer of HIV-1 promotes viral fusion. A better understanding of the consequences of cell-cell transmission of HIV-1 may lead to the development of antiretrovirals that are more effective in vivo. )